Monitoring parameters of a motor vehicle wheel, for example the tire pressure or the tire temperature, plays a decisive role in the safety of the vehicle or the driver. To dispense with the need for manual checking of these types of parameters devices have been developed with which it is possible to record the parameters of the wheels automatically and display them using the appropriate display devices on the dashboard for example. In such cases it is desirable not only to display the parameters for all the cases in which the allowable range of a parameter has been exceeded but also to specify a unique assignment between the displayed parameters and the position of the wheel concerned. This necessitates an arrangement of this type to be designed in such a way that this assignment will be correctly made even after a wheel has been changed.
As part of an assignment or initialization process, which is undertaken as required or at predetermined intervals, it is necessary to assign the identifier of each detector unit which is allocated to a wheel to the appropriate wheel position, for example “front left”, “front right”, “rear right”, “rear left”. In normal operating mode, the identifier, which is contained in a signal sent by one of the number of detector units can then be used as a basis for assigning the relevant wheel position, by the detected identifier being compared with the stored assignment information (the identifiers assigned to wheel positions are stored).
It would of course also be conceivable to execute this type of assignment process each time before a signal of a specific detector unit is interpreted. As a rule however this is too awkward and time-consuming.
A method of assigning transmitters to receiver antennas in tire pressure monitoring systems is known from EP-B-0 861 160, in which a pressure measurement sensor, a transmitter and a send antenna are each assigned to a wheel. In addition each wheel is assigned a receiver antenna on the bodywork which is connected via a cable in each case to the receiving and evaluation electronics. The assignment of the identifiers to wheel positions is made by a signal sent from a transmitter or from the associated transmit antenna being received by all receiver antennas and that the (wheel) position of the receiver antenna which delivers the signal with the greatest intensity is assigned to the corresponding transmitter and its identifier.
The disadvantage here is that each wheel position must be assigned a receiver antenna, which in its turn must be connected via a corresponding cable to the receiving and evaluation electronics. This brings corresponding installation effort and correspondingly high costs for the number of receiver antennas corresponding to the number of wheels.
A system for recording tire pressures is known from U.S. Pat. No. 5,774,047, in which a detector unit with a transmitter is also assigned to each wheel. The transmit signals are evaluated and assigned to the wheel positions by providing at least two receiver antennas, whereby the phase difference and polarity of the two receive signals in each case is evaluated, which is delivered by the at least two receiver antennas on receiving the signal of one and the same transmitter.
This type of evaluation of the polarity and phase relationships of the signals is however linked to considerable expense as regards the circuitry.